The present invention refers to diesel engines of the type having at least one cylinder defined in a engine block, a cylinder head having a bottom surface delimiting a combustion chamber associated to said cylinder, at least two intake valves and two exhaust associated to said cylinder, which control respective intake and exhaust conduits formed in said cylinder head and ending up on said bottom surface of the cylinder head delimiting the combustion chamber, wherein said surface is a mainly flat surface and orthogonal to the cylinder axis, and wherein said intake and exhaust valves each have a respective valve axis which is inclined with respect to the cylinder axis by an angle greater than 8°.
An engine of this type is known from document DE 101 50 721 A1 or document EP 1 835 143 A1.
In diesel engines of a more conventional type, intake and exhaust valves have their axis inclined with respect to the axis of the cylinder by an angle not greater than 3-4°. The surface of the head delimiting the combustion chamber is a flat surface (the so-called “flame plate”) substantially coplanar with the bottom face of the cylinder head. When the piston is at the top dead centre, the combustion chamber is almost entirely defined by a cavity (the so-called “bowl”) obtained in the head of the piston.
During the operation of the engine, the air motions—required to guarantee proper mixing of the diesel fuel injected into the combustion chamber and an ensuing ideal combustion—are characterized by a vortex with an axis parallel to the cylinder axis, referred to as “swirl”. Such swirl motion is indicated by the arrow SW in FIG. 1 of the attached drawings, which shows—in a plane orthogonal to the cylinder axis—the combustion chamber associated to the cylinder, indicated with reference number 1, into which the two intake conduits A1, A2, and the two exhaust conduits S1, S2 end up. According to an art known art per se, the first intake conduit A1 has a scroll-shaped end, suitable to impart a helical movement (indicated by the arrow C in FIG. 1) to the airflow while the second intake conduit A2 directs the airflow which reaches into the combustion chamber according to a direction tangential with respect to the axis of the combustion chamber. The abovementioned shape of the intake conduits is selected to generate the desired swirl motion SW. Furthermore, in the operation of the engine, during the stage of compression in the cylinder, in which the intake valves are closed, the vorticity of the air motion is determined by the so-called “spin-up” phenomenon due to the fact that the air vortex is pushed into the abovementioned “bowl” during the ascent of the piston. The reduction of the radius of the air vortex in the “bowl” causes, according to the principle of preservation of the quantity of motion, an increase of speed. The swirl is quantitatively represented by the “swirl ratio” (Rs=Ωeq z/Ωmot where Ωeq z is the equivalent rotational speed of the fluid vortex around cylinder axis, intended as the ratio between the angular momentum and the inertia momentum of the mass distribution, while Ωmot is the rotational speed of the driving shaft).
The variation of the swirl ratio in a conventional diesel engine during the intake and compression stages is illustrated in FIG. 2 in the attached drawings. The diagram in such figure shows that the achievement of the ideal value for the combustion, at 360° of crank angle, is mostly caused by the swirl motion generated by the shape of the intake conduits (see the part of the diagram comprised between 120° and 320°) and for a final part—corresponding to the final compression stage—by the “spin-up” phenomenon.
The “spin-up” is instead relatively low in the case of conventional petrol engines, in which the intake and exhaust valves usually have their axis considerably inclined with respect to the to the axis of the cylinder (according to an angle usually greater than 8°), hence the surface of the cylinder head delimiting the combustion chamber is not a coplanar flat surface at the bottom face of the head, but it is defined by a cavity which forms a considerable part of the volume of the combustion chamber when the piston is at the top dead centre.
In the case of petrol engines, the main air motions that contribute to the ideal turbulence for combustion are made up of a horizontal axis vortex referred to as tumble. FIG. 3 of the attached drawings shows, by way of reference, the abovementioned tumble motions in a cylinder of a petrol engine during the intake step (left part of the figure) and during the compression step (right part of the figure). The views of FIG. 3 are sections according to a dashed line also indicated in such figure, through the axes of an intake valve and of an exhaust valve. Furthermore, such views just illustrate the combustion chamber, i.e. the part comprised between the surface of the chamber defined by the cylinder head and the top surface of the piston (which is illustrated at a higher position in the right part of the figure).
The different inclination of the engine valves in diesel engines and in petrol engines gives rise to complications when constructing engines, hence complicating the joining of the components and the construction lines as well as assembling such engines.
The aim of the present invention is that of providing a new shape of the combustion chamber for diesel engines capable of allowing standardising—at least to a given extent—the architecture of such engines with that of petrol engines, so as to allow considerable simplification and reduction of costs when manufacturing and assembling such engines.
According to the invention, such object is attained due to the characteristics indicated in the attached claim 1. Further preferred and advantageous characteristics of the invention are indicated in the attached dependent claims.